This disclosure generally relates to optical measuring devices. More particularly, this disclosure relates to apparatus and methods for optically measuring the angular position of a rotating part.
Optical metrology systems measuring dimensions in three degrees of freedom are limited in their accuracy by the high uncertainties in the multi-axis pointing angle measurements of the optical measurement system. Gimbals are multi-axis line-of-sight pointing devices typically used. Such gimbals are composed of rotation stages which have rotary encoders to report the angle the rotation stage occupies. These encoders set the limit for the quality of the overall three-dimensional measurement which the metrology system reports. For large-scale objects, such as airplanes, the impact of angle error dominates the uncertainties, limiting the range of applications of existing capability, and also incurring increased costs to meet quality verification objectives on large complex structures.
Existing solutions use techniques to encode the angles into electrical signals that are intrinsically limited in accuracy, and are not directly traceable to any length, time, or other basic standard. Generally, existing rotary encoders utilize a structure with encoding that presents a pattern to a pattern sensor, which senses changes in the code as a function of angle of the rotating object. A typical encoder is a series of linear regions arrayed along the radii of a circle of fixed diameter. An optical sensor measures the change in intensity when the pattern is illuminated. Lack of traceability to standards causes measurements in different parts of the world to be different. While it is possible to directly calibrate a laser tracker's range performance to a national standard (e.g., frequency), a tracker's three-dimensional performance is limited by the angle measurements, for which there is no directly traceable measurement.
It would be advantageous to provide encoders which report rotation angle measurements that are directly traceable to a standard.